This invention relates to a magnetoresistive effect element, magnetic memory and magnetic head, and more particularly to those having a ferromagnetic tunnel junction structure and capable of maintaining high sensitivity to an external magnetic field even when miniaturized in device size.
Magnetoresistive effect elements are under expectation toward practical use in a wide field of application including magnetic detector elements such as magnetic heads, magnetic memory devices, etc.
For example, there is a proposal of magnetic random access memory using a magnetic element exhibiting giant magnetoresistance effect as a solid magnetic storage device. Especially, magnetic memory using “ferromagnetic tunnel junction” is remarked as a magnetic element.
Ferromagnetic tunnel junction is mainly made of a three-layered film of first ferromagnetic layer/insulating film/second ferromagnetic layer, and a current flows, tunneling through the insulating film. In this case, the junction resistance value varies proportionally to the cosine of the relative angle between magnetization directions of the first and second ferromagnetic layers. Therefore, resistance value becomes minimum when the magnetization directions of the first and second ferromagnetic layers are parallel, and becomes maximum when they are anti-parallel. This is called tunneling magnetic resistance (TMR) effect. For example, in the literature, Appl. Phys. Lett., Vol. 77, p 283 (2000), it is reported that changes of resistance value by TMR effect reaches as high as 49.7% at the room temperature.
In a magnetic memory including a ferromagnetic tunnel junction as a memory cell, magnetization of one of ferromagnetic layers is fixed as a “reference layer”, and the other ferromagnetic layer is used as a “recording layer”. In this cell, by assigning parallel and anti-parallel magnetic orientations of the reference layer and the recording layer to binary information “0” and “1”, information can be stored.
For writing information, magnetization of the recording layer is reversed by a magnetic field generated by supplying a current to a write line provided for the cell, and by detecting a resistance change by TMR effect. A number of such memory elements are aligned to form a large-capacity memory device.
Its actual configuration is made up by placing a switching transistor for each cell and combining peripheral circuits similarly to DRAM (dynamic random access memory), for example. There is also a proposal of a system incorporating ferromagnetic tunnel junctions in combination with diodes at crossing positions of word lines and bit lines (U.S. Pat. No. 5,640,343 and U.S. Pat. No. 5,650,958).
For higher integration of magnetic memory elements using ferromagnetic tunnel junctions as memory cells, the size of each memory cell becomes smaller, and the size of the ferromagnetic element forming the cell inevitably becomes smaller. There is the same situation in magnetic recording systems when the recording density is enhanced and the recording bit size is decreased.
In general, as the ferromagnetic element becomes smaller, its coercive force increases. Since the intensity of the coercive force gives criteria for judging the magnitude of the switching magnetic field required for reversal of magnetization, its increase directly means an increase of the switching magnetic field. Therefore, upon writing bit information, a larger current must be supplied to the write line, and it invites undesirable results such as an increase of power consumption, shortening the wiring lifetime, etc. Therefore, it is an important issue for practical application of high-integrated magnetic memory to reduce the coercive force of the ferromagnetic element used as the memory cell of magnetic memory.
To overcome this problem, it has been proposed to use, as a “recording layer”, a structure including multi-layered film of at least two ferromagnetic layers and a nonmagnetic layer interposed between them and including anti-ferromagnetic coupling between those ferromagnetic layers (Japanese Patent Laid-Open Publication No. H9-25162, Japanese Patent Application No. H11-263741 and U.S. Pat. No. 5,953,248).
In this case, two ferromagnetic layers included in the “recording layer” are different in magnetic moment and thickness, and their magnetic orientations are opposite under anti-ferromagnetic coupling. Therefore, they effectively cancel each other's magnetization, and the entirety of the recording layer can be regarded equivalent to a ferromagnetic element having small magnetization in the easy axis direction. If a magnetic field is applied in the opposite direction from orientation of the small magnetization in the easy axis direction the recording layer has, magnetization of each ferromagnetic layer reverses while holding the anti-ferromagnetic coupling. Therefore, because of the closed magnetic line of force, influences of the demagnetizing field are small, and the switching magnetic field of the recording layer is determined by the coercive force of each ferromagnetic layer. As a result, even a small switching magnetic field enables magnetic reversal.
In case that no layer-to-layer coupling exists between the magnetic layers (J=0), there is an interaction by magnetostatic coupling by the leak magnetic field from the magnetic layers. In this case, however, it is known that the switching magnetic field decreases similarly to a case where such coupling exists (24th Japan Applied Magnetics Academy Scientific Lecture 12aB-3, 12aB-7, 24th Japan Applied Magnetics Academy Scientific Lecture Summary p. 26, 27).
However, in case that only magnetostatic coupling exists without no layer-to-layer coupling between magnetic layers, the magnetic structure made by the above-explained magnetization is unstable. Additionally, the squareness in the hysteresis curve or the magnetoresistance curve is small, and it is difficult to obtain a large magnetoresistance ratio. Therefore, it is not preferable for use as a magnetoresistive.
As explained above, reducing the switching magnetic field necessary for magnetic reversal of the “recording layer” is an indispensable factor for realization of a high-density magnetic recording system or magnetic memory, and it has been proposed to use a multi-layered film including anti-ferromagnetic coupling through a nonmagnetic metal layer.
However, as already recognized, in a minute ferromagnetic element in a minute magnetoresistive effect element as used in a high-density magnetic recording system or high-integrated magnetic memory, when the width of its shorter axis is miniaturized to the level of several microns through sub microns, a magnetic structure different from a central portion of the magnetic element is generated in perimeter portions of the magnetized region due to influences of an “demagnetizing field”. Such a magnetic structure in perimeters is called “edge domain” (see, for example, J. App. Phys., 81, p. 5471 (1977)).
FIGS. 18A and 18B are schematic diagrams showing magnetic structures having such edge domains, respectively. In any of the magnetic structures shown in FIGS. 18A and 18B, magnetization M1 is generated in a direction in accordance with the magnetic anisotropy in a central portion of the magnetized region. In opposite end portions, however, magnetizations M2 through M5 are generated in directions different from that of the central portion. In this explanation, the domain structure shown in FIG. 18A is called “S-type structure”, and the domain structure shown in FIG. 18B is called “C-type structure”.
In a minute magnetic element used in a high-density magnetic recording system or high-integrated magnetic memory, the edge domain generated in its end portions exert strong influences, and changes of the magnetic structure pattern upon magnetic reversal becomes complicate. As a result, the coercive force increases, and the switching magnetic field undesirably increases.
As a method for minimizing such complicate changes of the magnetic structure, there is a proposal to fix the edge domain (U.S. Pat. No. 5,748,524, Japanese Patent Laid-Open 2000-100153). This method can certainly control behaviors upon magnetic reversal, but cannot substantially reduce the switching magnetic field. Additionally, his method needs an additional structure for fixing the edge domain, and it is not suitable for higher-density applications.